A Another loophole-free test of Bell's theorem

[.Scott]

If you believe in the many world theory then (it seems to me) you do not believe in any ordinary reality at all. There are no "measurement outcomes". Not ever. They are somehow illusory. Many world theorists do not have any problem with non-locality because there is no classical world where measurements actually have outcomes.

I am hardly a subscriber to MWI, but if we are considering any loophole what-so-ever, MWI does introduce a somewhat bizarre (at least in human terms) loophole to the Bell experiments. If we assume that once Bell-compliant measurements are made that the Bell-compliant result can cause likely doom to the measurement results, then our ability to examine the results as posted in arxiv is evidence that we are in a select MWI world where we lucked out because of the atypical, non Bell-supportive results.

I don't think this can be completely dismissed as philosophical. For example, if at some point we determine that the Higg's particle is completely unstable, then this would provide a model for how we could persist in an inherently unstable universe. So the MWI/Doom theory states that bizarrely good luck is an inherent trait of the universe - and there are conditions where that could be experimentally determined.

I suspect there are other interpretations (non-MWI) that would yield the same evidence - and create the same loophole for Bell experiments.

On the other hand, would MWI be considered a violation of local reality theory? I don't think so. It is usually described in terms of what causes an event - and no discussion it provided on what happens after that event.

 

[akhmeteli]

First, any interpretation of any experiment in nature is based on an assumption that, strictly speaking, is not correct. Physics (unlike mathematics) is a FAPP science.

Generally, maybe, but I guess it does depend on the task we are trying to solve. In this case, I believe, we are trying to decide, based on the results of experiments, if, say, Nature is (non)local. I cannot imagine how Nature can be nonlocal or local "for all practical purposes". What does approximate locality mean? I have no idea.

Second, just because outcomes are not fixed does not mean that they are not definite. For instance, the shape of a flag on wind is definite but not fixed. This means that the shape changes with time (so is not fixed), but at each given time the flag has some definite shape.

You probably have in mind the picture of Bohmian mechanics where there is a wave function, on the one hand, and a point particle somewhere there, on the other hand. This may be a correct picture of reality, or an incorrect picture, I don't know. However, I am not sure we can determine the position of that point particle, when we conduct a measurement, we just have unitary evolution in Bohmian mechanics, nothing more, as far as I can see, and unitary evolution cannot definitely determine the true position of the point particle, as far as I understand. Furthermore, let us leave alone definite outcomes for a moment. If you agree that the outcomes are not fixed, then the assumption used for interpretation of the new experiments is, strictly speaking incorrect.

 

[akhmeteli]

Yes, in that sense, macroscopic superpositions imply many-worlds. The many worlds in MWI are different states of the same universe. So if there is a possible state of the world in which (say) Al Gore won the election in 2000, and a state of the world in which George Bush won, then you can form a superposition of the two states. MWI is about such macroscopic superpositions. The "many" refers to many different macroscopically distinguishable states that exist at the same time as elements of a superposition.

I don't see how macroscopic superpositions imply many-worlds. I can (and want to) take the following position: macroscopic superpositions, if unitary evolution predicts them, exist just in one world, ours. A superposition of two different US presidents or the Schroedinger's cat may seem crazy, but I don't know how you can bury such a position without burying unitary evolution in the same grave.

 

[akhmeteli]

Could you elaborate/explain " no fixed outcomes " Is this related to non realism or non deterministic hidden variables
or related to detector /particle interactions in context of inequality violations ?

It is natural to believe that measurement in quantum mechanics can be described as unitary evolution of the larger system including the initial system under measurement and the instrument. It is well-known that unitary evolution cannot, say, turn a pure state into a mixture, that means that, strictly speaking, we can never (or hardly ever) have fixed outcomes of measurement. This is one of the aspects of the measurement problem of quantum theory. So how is this related to the recent experiments on the Bell inequalities violation? The interpretation of all of these experiments assumes fixed outcomes at some point. However, unitary evolution of quantum theory does not allow such fixed outcomes. Therefore, it looks like the conclusion of the experiments is: local realistic theories are wrong, assuming that quantum theory is wrong. So we cannot bury local realistic theories using the results of these experiments without burying quantum theory in the process. This seems a little bit confusing:-)

 

[nikman]

The interpretation of all of these experiments assumes fixed outcomes at some point. However, unitary evolution of quantum theory does not allow such fixed outcomes. Therefore, it looks like the conclusion of the experiments is: local realistic theories are wrong, assuming that quantum theory is wrong. So we cannot bury local realistic theories using the results of these experiments without burying quantum theory in the process.

How is it necessary to assume wrongness of quantum theory in connection with the experiments? The measurement "collapses the wavefunction" rendering it no longer unitary and yields an eigenvalue. The eigenvalue is a classical artifact. So, in effect all measurements are classical. We're exploring the quantum domain using classical concepts and classical tools and the results we obtain are classical, not quantum, results.

As Wheeler once put it, what we "know" is analogous to a landscape of tops of fence-posts sticking up out of snowdrifts. BT doesn't say that nonlocality and/or nonrealism is or are objective truths -- it says that no model which assumes local realism can replicate the predictions of QM. The results of the Bell experiments demonstrate that EPR can't prove their contention that quantum theory is incomplete. They set the rules; they lose the game.

 

[akhmeteli]

How is it necessary to assume wrongness of quantum theory in connection with the experiments? The measurement "collapses the wavefunction" rendering it no longer unitary and yields an eigenvalue. The eigenvalue is a classical artifact. So, in effect all measurements are classical. We're exploring the quantum domain using classical concepts and classical tools and the results we obtain are classical, not quantum, results.

First of all, according to M. Schlosshauer (Annals of Physics, 321 (2006) 112-149), "no positive experimental evidence exists for physical state-vector collapse". The same source states that "the universal validity of unitary dynamics and the superposition principle has been confirmed far into the mesoscopic and macroscopic realm in all experiments conducted thus far". Third, the von Neumann's picture that you use (unitary evolution between measurements, collapse during measurements) seems a bit strange: if you call something "a measurement", you get one result, if you don't call the same process "a measurement", you get a different result (using unitary evolution of the larger system including the instrument). Of course, you can say that this is actually perfectly OK, and there is no measurement problem in quantum mechanics, but not everybody agrees. And let me repeat this: there is no experimental evidence of failure of unitary evolution.
We can look at this from a different point of view. OK, let us imagine for a moment that we chose a quantum theory according to von Neumann: it is unitary evolution between measurements, it is collapse during measurements. Very well. Then the recent experiments seem to show that local realistic theories are wrong assuming fixed outcomes, therefore assuming collapse. So we have to choose: either local realistic theories are wrong, or collapse is wrong. For you that means that local realistic theories are wrong, but I am free to think that collapse is wrong, as there is no experimental evidence of collapse. Therefore, in my book, the recent experiments still have a loophole.

As Wheeler once put it, what we "know" is analogous to a landscape of tops of fence-posts sticking up out of snowdrifts. BT doesn't say that nonlocality and/or nonrealism is or are objective truths -- it says that no model which assumes local realism can replicate the predictions of QM. The results of the Bell experiments demonstrate that EPR can't prove their contention that quantum theory is incomplete. They set the rules; they lose the game.

I agree, no local realist theories can replicate the predictions of QM, if you include collapse in QM. But there is no evidence of collapse, so, as of now, local realistic theories do not need to emulate collapse to be in agreement with experiments.

 

[nikman]

And let me repeat this: there is no experimental evidence of failure of unitary evolution.

Don't know what this means. I put "collapses the wavefunction" in scare quotes for a reason I assumed was obvious. Bohr for example considered state vector collapse a mathematical convenience possessing no physical reality. Dieter Zeh and the Many Minds school disown collapse as conventionally understood; there's seamless evolution into decoherence with a substrate of coherence apparently preserved. But everyone agrees that measurement results in something important happening. Ordinarily it relates to decoherence in one form or another, or else incoherence. In quantum information theory a single particle carries one bit of coherent classical information. You measure the particle and extract that bit and then measure the particle again and find you receive incorrect data if not irreducible randomness which some would simply call gibberish. But from whatever direction you approach it, measurement is life-changing. Unitary evolution is disturbed or disrupted or destroyed; so why not say it fails.

 

[akhmeteli]

Don't know what this means. I put "collapses the wavefunction" in scare quotes for a reason I assumed was obvious. Bohr for example considered state vector collapse a mathematical convenience possessing no physical reality. Dieter Zeh and the Many Minds school disown collapse as conventionally understood; there's seamless evolution into decoherence with a substrate of coherence apparently preserved. But everyone agrees that measurement results in something important happening. Ordinarily it relates to decoherence in one form or another, or else incoherence. In quantum information theory a single particle carries one bit of coherent classical information. You measure the particle and extract that bit and then measure the particle again and find you receive incorrect data if not irreducible randomness which some would simply call gibberish. But from whatever direction you approach it, measurement is life-changing. Unitary evolution is disturbed or disrupted or destroyed; so why not say it fails.

Maybe measurement results in something important, maybe it does not, but there is no experimental evidence of a failure of unitary evolution. What you say about this bit measurement is not a proof of the opposite. The particle has interacted with the instrument, no wonder that it is much more difficult to observe coherence in the enlarged system including the instrument. The same is true about decoherence. If you cannot find coherence, that does not mean it disappeared. Each time you can fully control your system, even if it consists of hundreds of particles, you see coherence. If you cannot control your system, you cannot blame unitary evolution. There is no evidence of failure of unitary evolution, so why say it fails? Again, you may believe that it fails, but I am under no obligation to believe that until experiments show such failure.

 

[DrClaude]

Unfortunately, my warning wasn't heeded, and this thread is no longer about the the test of Bell's inequality, but rather a fruitless discussion.

Thread closed.